Nerissa joined the hunters as an adult. She had escaped from an abusive boyfriend (she's bisexual) to a women's shelter that was also a cover for huntress activities. Unfortunately, her old relationship pattern is repeating with Addy, who is a bit unstable and had trouble with the word "No."
She doesn't have the muscle, training, or scientific background to assist her newfound family in their standard avenues, but they tend to accept whatever their members do well that can help. In Nerissa's case, this means foraging literature for clues of historical undead activity. She is also sometimes a day huntress (staking or baking vampires in their daytime shade), as opposed to a night huntress (taking out the mofos while they try to draw prey -- Jaina's job).